1. Field of the Invention
This invention relates to a developing roller and a developing assembly which are used in, e.g., electrophotographic apparatus such as copying machines and laser beam printers.
2. Related Background Art
Conventionally, in electrophotographic apparatus or electrostatic recording apparatus, such as copying machines and laser beam printers, a pressure developing method is known as a developing method, in which a non-magnetic one-component developer is fed to, e.g., a photosensitive drum which is holding a latent image thereon and the developer is made to adhere to the latent image (electrostatically charged image) to render the latent image visible. According to this method, any magnetic material is not required, and hence the image forming apparatus can be easily simplified and miniaturized or toners can be easily made into color toners.
In this developing method, a developing roller holding a toner (non-magnetic one-component developer) thereon is brought into contact with a latent image bearing member holding an electrostatic latent image thereon, such as a photosensitive drum, to attach the toner to the latent image to perform development. Hence, the developing roller must be formed of a conductive elastic member.
This method in which the developing roller is brought into contact with the photosensitive drum to develop the latent image with the toner is necessarily required to ensure the uniformity of a toner layer on the developing roller and the charging uniformity of the toner. However, in recent years, with the increase of printing speed and the improvement of image quality, the precision required of the developing roller has become increasingly severer. As one thing therefor, depending on the precision of engagement of gears which drive the developing roller, the toner coat layer may be disordered with gear pitches, and the charging uniformity of the toner also is disordered in a short period, so that horizontal lines due to such gear pitches (hereinafter “gear pitch horizontal lines”) appear on images, which has come into question. As related background art, Japanese Patent Application Laid-Open No. 2001-042631 discloses a developing roller having an elastic layer. However, in rollers constituted commonly, if an elastic layer is merely covered thereon with a surface layer formed of a resin, the developing roller may have a much higher hardness than the hardness of the elastic layer itself, so that it may be difficult to solve the problem of the gear pitch horizontal lines.